Modelling and enhancing track support through railway switches & crossings

Railway turnouts, also known as Switches and Crossings (S&Cs),are important systems in the railway network that enable railway vehicles to change routes. Nevertheless, severe impact loads occur when the vehicle negotiates a turnout due to the wheel load transfer from one rail to another, which leads to high maintenance costs. In turn, the design of S&Cs is currently a complex task for which the development of accurate computational models represents a great value not only to better investigate the vehicle-turnout interaction but also to support the design of turnouts.

This thesis proposes a complete computational study that includes: the development of a detailed model of a railway crossover; model calibration against site measurements; performance assessment of the crossover; and proposing alternative designs that enhance the resistance to damage.

Different track models are compared to select the most suitable one to study S&Cs. A UK site with Under Sleeper Pads (USPs) is the case study considered in this thesis and a numerical model is built using the track model selected. Two measurement campaigns, one performed in 2016 and the other in 2019, are used in this work, comprising bearer displacements measured at the switch and crossing panels. Every 2 bearers on the cess side and a few bearers on the 6ft side of the track were recorded on the first campaign. The model trackbed stiffnesses are calibrated against the site measurements and a good agreement is achieved. The data from the second campaign showed an increase in displacements that most likely indicate the existence of voided bearers. Also, from observations in situ and photos taken from the site, it was possible to assume that some of the connection devices between bearers of the two tracks were damaged. This required changes to be made on the proposed model developed, including the consideration of bearer flexibility and the use of non-linear force elements to model the varying trackbed stiffness and accommodate voids underneath the bearer. A new calibration of the model is performed but reducing the trackbed stiffness was not enough to reach the level of displacements measured. An incremental process of the model development is then used, considering voids under the bearers, removing the connection from bearers where loose bolts were spotted and considering some level of flexibility at the other connections rather than assuming they are rigid as initially intended. Finally, the reduction of the trackbed stiffness of the second track was included to simulate the hypothesis of voids being present at that track. From all these changes, the voided bearers and the low stiffness of the connection are the ones that lead to the biggest increase of the displacements.

The calibrated model is then used to investigate the effect of different turnout characteristics, such as, the variation on track stiffness due to the S&C components, the use of USPs and alternative bearer connections. Thus, several numerical simulations of a typical railway vehicle running over a turnout have been carried out. Here, special attention has been given to the wheel-rail contact forces, which leads to wear of the rails, to the reaction forces on the bearer connection, which leads to its failure, and to the forces transmitted to the ballast, which leads to track settlement. The choice of installing two different types of USPs on the site has been investigated and concluded that they do not offer an ideal smoothing of the track stiffness. Yet, they reduce the ballast forces, particularly in the areas with the higher transient loads. Regarding the bearer connections, it is concluded that high reaction forces occur around the crossing nose and the load transfer area at the switch panel, which justifies their failure. Two alternative connection designs are studied, namely the “pinned” and the “sandwich” joints. It has been concluded that the sandwich joint reduces the stresses on the connection device and thus would improve its reliability, although higher displacements would occur, which can lead to faster development of voids.